1. Field of the Invention
The present invention relates to a toner capable of forming high quality images, for developing latent electrostatic images in electrophotography, electrostatic recording, electrostatic printing and the like. More specifically, the present invention relates to a toner which is excellent in chargeability while maintaining low temperature fixability, and heat resistant storage stability and which is also excellent in permeability (OHP permeability, etc.), a method for producing the same, and a developer using the toner.
2. Description of the Related Art
In electrophotographic apparatuses and electrostatic image recording apparatuses, an electric or magnetic latent image is visualized using a toner. For example, in electrophotography, a latent electrostatic image (latent image) is formed on a photoconductor, an electrostatic image (latent image) is formed on a photoconductor, and then the electrostatic image is developer using a toner to thereby form a toner image. Such a toner image is usually transferred onto a transfer material such as paper, and then fixed on the transfer material by a method such as heating. A toner for use in developing is typically colored particles containing a coolant, a charge controlling agent and other additives in a binder resin. The method of producing a toner is broadly classified into the pulverization method and the suspension polymerization method. In the pulverization method, a colorant, a charge controlling agent, an offset preventing agent and the like are melt-mixed in a thermoplastic resin, and the resulting composition is pulverized and classified to thereby producing a toner. According to the pulverization method, a toner which is excellent in properties to a certain degree can be produced, however, there is a limitation in selection of toner materials. For example, a toner composition obtained by melt-mixing needs to be pulverized and classified by a economically usable device. For this reason, the particle size distribution thereof tends to be wider, and to obtain a copy image having a high resolution and a high gray scale, for example, it is necessary to remove fine power particles having a particle diameter of 5 μm or smaller and coarse powder particles having a particle diameter of 20 μm or greater, and thus the pulverization method has a disadvantage in that the yield becomes very low. In addition, in the pulverization method, it is difficult to uniformly disperse a colorant, a charge controlling agent and the like in a thermoplastic resin. Further, the method has such a problem that a colorant is exposed on a surface of toner, and charges on the surface of the toner become nonuniform, thereby causing degradation in developing properties.
Recently, to overcome these problems in the pulverization method, there have been proposed and implemented toner production methods by a suspension polymerization method. However, such a known polymerization method, for example, a toner obtained by a suspension polymerization method, has a spherical shape and has a drawback in poor cleanability. A toner produced by the suspension polymerization method causes less untransferred toner and is unlikely to cause cleaning failure in a developing/transferring process using an image having low image-occupation-area ratio, however, in the case of a high image-occupation-area ratio, such as a photographic image, or when toner forming an untransferred image due to paper feeding failure or the like remains as untransferred toner and the untransferred toner accumulates, background smear of image occurs. Further, there is a probability that the untransferred toner contaminates a charging roller which contacts and charges the photoconductor, and the charging roller may not exhibit its inherent chargeability. Further, since a toner is polymerized in the production of the toner, in most cases, materials that have been conventionally used for toner cannot be used. Even when such conventional toner materials can be used, the resulting toner is influenced by additives such as a resin and a colorant contained therein, the particle size thereof may not be sufficiently controlled, and thus a toner produced by the suspension polymerization has low freedom degree concerning.
To solve the problems, there has been proposed a method in which resin particles obtained by an emulsification polymerization method are made associated with each other to obtain an indefinite shape toner (Japanese Patent (JP-B) No. 2537503). However, in a toner obtained by an emulsification polymerization method, polyester resins, which exhibit excellent fixability and color suitability in kneading/pulverization method cannot be basically used, and the emulsification polymerization methods have drawbacks of their incapabilities of meeting the requirements of compactness of apparatus, high-speeding performance, and color suitability.
Then, the present applicant proposed a method of obtaining a toner by a dissolution suspension method, using, as a toner material, a polyester-modified resin obtained by reacting a polyester resin precursor which is excellent in transparency, mechanical strength and low temperature fixability (Japanese Patent (JP-B) No. 3640918). With this, such a toner is obtained which can solve the above-mentioned problems, is usable in low temperature fixing system while maintaining high cleanability, excellent in offset resistance and capable of preventing smear on fixing devices and images. However, a toner produced by the dissolution suspension method of JP-B No. 3640918 has a difficulty in dissolving a colorant therein with uniformity and tends to cause localization of colorants (pigments) on toner particle surfaces and a difference of the colorant content in each toner, and thus nonuniformity of charges occurs, leading to degradation in charge stability when used for a long time. In addition, in the case of outputting a color image, slight degradation of developing ability and transferability causes degradation in color balance and gray scale. Further, a colorant in a toner is typically incompatible with resins, and thus it has disadvantages in that when a colorant is poorly dispersed, the toner causes scattering or reflection of transmitted light at its surface boundary, impairing the permeability to OHP and the like.
In the light of the circumstances described above, the present applicant proposed various methods for obtaining a toner by the dissolution suspension method. (see Japanese Patent Application Laid-Open (JP-A) Nos. 2007-94351, 2007-248746, 2007-94352, 2006-293304 2005-70187, 2007-248979, 2008-76453 and 2006-18018).
For example, as a method for obtaining a yellow toner which is excellent in offset resistance, chargeability, storage stability, color formability, tinting strength and heat resistance and has OHP-permeability, the following methods have been proposed: a method of controlling a shape factor using a binder resin and a colorant having a specific structure (Japanese Patent Application Laid-Open (JP-A) No. 2007-94351), and a method of using a specific colorant which is masterbatch processed by kneading with a resin constituting a binder resin (Japanese Patent Application Laid-Open (JP-A) No. 2007-248746). In addition, the present applicant proposed a method of controlling the shape factor of a toner using a colorant having a specific structure, and a dispersant made of an acryl-based polymer having N,N-substituted diamino group and an acidic group (Japanese Patent Application Laid-Open (JP-A) No. 2007-94352). As a method of obtaining a toner in which the dispersibility of a colorant is improved, the present applicant proposed a method of using a dispersant in which the acid value and the amine value are specified (Japanese Patent Application Laid-Open (JP-A) No. 2006-293304). As a method of obtaining a toner capable of maintaining cleanability, responding to low-temperature fixing systems, hand having excellent offset resistance without smearing fixing devices and images, the present applicant proposed a method of granulating toner materials in an aqueous medium containing resin fine particles and inorganic fine particles to thereby make the resin fine particles remain on surfaces of toner particles (Japanese Patent Application Laid-Open (JP-A) No. 2005-70187). Also, as a method of obtaining a toner capable of obtaining high quality images while maintaining the uniformity of the composition, and charging stability but having less fogging and toner scattering, and having small particle diameters and a narrow particle size distribution, the present applicant proposed a method of increasing the volume average particle diameter of a water-in-oil emulsion type dispersion liquid formed in an aqueous medium (Japanese Patent Application Laid-Open (JP-A) No. 2007-248979). Further, as a method of obtaining a toner capable of simultaneously achieving both low temperature fixability and offset resistance and forming high fine images, the present applicant proposed a method of specifying solubility parameters of two or more binder resins (Japanese Patent Application Laid-Open (JP-A) No. 2008-76453). Furthermore, as a method of obtaining a toner ensuring the low temperature fixability, offset resistance and color reproducibility, the present applicant proposed a method of using a colorant which is obtainable by dispersing a pigment in resin components, employing a binder resin containing a crystalline polyester resin and a flashing method (Japanese Patent Application Laid-Open (JP-A) No. 2006-18018).
With the methods described in JP-A Nos. 2007-94351, 2007-248746, 2007-94352, 2006-293304, 2005-70187, 2007-248979, 2008-76453, and 2006-18018, certain effects for solving the above problems are obtained, however, demands for more sophisticated improvement in chargeability and improvement in permeability (OHP etc.) arise, and it is desired to improve the dispersibility of colorants.
Note that as a method of obtaining a yellow toner excellent in chargeability of positive charge, there has been proposed a method of using an acrylic resin, a colorant and an amine compound in the dissolution suspension method (Japanese Patent Application Laid-Open (JP-A) No. 2009-57399). In addition, as a method of obtaining a yellow toner, a method is proposed in which a dispersion containing a resin material, a colorant and an organic solvent is dispersed in an aqueous dispersion medium by a dissolution suspension method, to prepare a dispersion liquid, and the dispersion liquid is uniformly formed (Japanese Patent Application Laid-Open (JP-A) No. 2008-203370).
However, it cannot be said that any of these proposals satisfactorily meet are adequately responsive to the demands for the dispersibility of colorants which are further improved in quality.
In granulation through the above mentioned polymerization method, not only water but also various materials such as a solvent, a surfactant, and a dispersion stabilizer, and thus the required technical level concerning the stability of conventional materials is further raised.
Particularly when toner particles are granulated, the dispersibility of a pigment and a releasing agent in a resin which has been dissolved or dispersed in a solvent greatly influences the fixing temperature range, color reproducibility range and developing properties of the resulting toner, and thus the dispersibility is one of the most important points in a toner production method through granulation of materials in an aqueous medium.
In particular, when Color Index No. PR122 (which may be abbreviated as PR122 hereinbelow) is used for a magenta toner and Color Index No. PY74 (which may be abbreviated as PY74 hereinbelow) is used for a yellow toner, the pigments are localized on the surface of toner because of the intrinsic properties of the pigments, causing a poor dispersed state.
As a result, there is such a problem that the dispersion state of the pigment in toner particles, i.e., the absorption spectrum intensity of the dispersion liquid is lowered as compared with favorably dispersed ones, and the resulting toner becomes inferior in the degree of pigmentation and the chromaticity to those expected.
As a countermeasure against the problem, it has been known that a pigment can be uniformly dispersed in toner using a pigment dispersant and thereby it is possible to produce a toner which is improved in the degree of pigmentation and the chromaticity (see Japanese Patent (JP-B) No. 4079257 and Japanese Patent Application Laid-Open (JP-A) No. 2009-116313).
However, even when the problem is solved and the dispersibility of a colorant is improved depending on a pigment dispersant used, a resin contained in the toner may be considerably lowered in viscosity and in its melting point by adding the pigment dispersant, and the produced toner may be solidified in a container during transportation.
Further, Japanese Patent (JP-B) No. 4213067 discloses a toner in which a crystalline polyester resin is incorporated into a binder resin, and thereby both the heat resistant storage stability and the low temperature fixability are simultaneously achieved.
However, crystalline polyester resins are sparingly soluble in solvents, the pigment cannot be dispersed in a solvent, and thus it is difficult to improve the degree of pigmentation and the chromaticity of the resulting toner.
For this reason, it has been required to disperse such a pigment in a solvent without reducing the viscosity and melting point of a resin used and to design a toner having a high degree of pigmentation, a wide color reproducibility range and excellent in storage stability.